Fix broken dartdoc.

utils/markdown/ast.dart

Index: utils/markdown/ast.dart
diff --git a/utils/markdown/ast.dart b/utils/markdown/ast.dart
index 361c7541b4f0dcbad5844eaec5638ca7a344590a..70e7f27b194e834fbecf05fa3fad41d2599abcab 100644
--- a/utils/markdown/ast.dart
+++ b/utils/markdown/ast.dart
@@ -15,19 +15,19 @@ class Element implements Node {
   final Map<String, String> attributes;
 
   Element(this.tag, this.children)
-    : attributes = <String, String>{};
+    : attributes = <String>{};
 
   Element.empty(this.tag)
     : children = null,
-      attributes = <String, String>{};
+      attributes = <String>{};
 
   Element.tag(this.tag)
     : children = [],
-      attributes = <String, String>{};
+      attributes = <String>{};
 
   Element.text(this.tag, String text)
     : children = [new Text(text)],
-      attributes = <String, String>{};
+      attributes = <String>{};
 
   bool get isEmpty() => children == null;